Plants which synthesize a modified starch, process for the production thereof and modified starch

ABSTRACT

Nucleic acid molecules are described encoding a starch granule-bound protein as well as methods and recombinant DNA molecules for the production of transgenic plant cells and plants synthesizing a modified starch with modified viscosity properties and a modified phosphate content. Moreover, the plant cells and plants resulting from those methods as well as the starch obtainable therefrom are described.

The present invention relates to nucleic acid molecules encoding a starch granule-bound protein as well as to methods and recombinant DNA molecules for the production of transgenic plant cells and plants synthesizing a modified starch with modified properties of viscosity and a modified phosphate content. The invention also relates to the transgenic plant cells and plants resulting from these methods and to the starch obtainable from the transgenic plant cells and plants.

The polysaccharide starch, which constitutes one of the most important storage substances in plants, is not only used in the area of foodstuffs but also plays a significant role as a regenerative material in the manufacturing of industrial products. In order to enable the use of this raw material in as many areas as possible, it is necessary to obtain a large variety of substances as well as to adapt these substances to the varying demands of the processing industry.

Although starch consists of a chemically homogeneous basic component, namely glucose, it does not constitute a homogeneous raw material. It is rather a complex mixture of various types of molecules which differ from each other in their degree of polymerization and in the degree of branching of the glucose chains. One differentiates particularly between amylose-starch, a basically non-branched polymer made up of α-1,4-glycosidically branched glucose molecules, and amylopectin-starch which in turn is a mixture of more or less heavily branched, glucose chains. The branching results from the occurrence of α-1,6-glycosidic interlinkings.

The molecular structure of starch which is mainly determined by its degree of branching, the amylose/amylopectin ratio, the average chain-length and the occurrence of phosphate groups is significant for important functional properties of starch or, respectively, its aqueous solutions. Important functional properties are for example solubility of the starch, tendency to retrogradation, capability of film formation, viscosity, colour stability, pastification properties, i.e. binding and gluing properties, as well as cold resistance. The starch granule size may also be significant for the various uses. The production of starch with a high amylose content is particularly significant. Furthermore, modified starch contained in plant cells may, under certain conditions, favorably alter the behavior of the plant cell. For example, it would be possible to decrease the starch degradation during the storage of the starch-containing organs such as seeds and tubers prior to their further processing by, for example, starch extraction. Moreover, there is some interest in producing modified starches which would render plant cells and plant organs containing this starch more suitable for further processing, such as for the production of popcorn or corn flakes from potato or of French fries, crisps or potato powder from potatoes. There is a particular interest in improving the starches in such a way, that they show a reduced “cold sweetening”, i.e. a decreased release of reduced sugars (especially glucose) during long-term storage at low temperatures. Specifically potatoes are often stored at temperatures of 4-8° C. in order to minimize the degradation of starch during storage. The reducing sugars released thereby, in particular glucose, lead to undesired browning reactions (so-called Maillard reactions) in the production of French fries and crisps.

Starch which can be isolated from plants is often adapted to certain industrial purposes by means of chemical modifications which are usually time-consuming and expensive. Therefore it is desirable to find possibilities to produce plants synthesizing a starch the properties of which already meet the demands of the processing industry.

Conventional methods for producing such plants are classical breeding methods and the production of mutants. Thus, for example, a mutant was produced from maize synthesizing starch with an altered viscosity (U.S. Pat. No. 5,331,108) and a maize variety (waxy maize) was established by means of breeding the starch of which consists of almost 100% amylopectin (Akasuka and Nelson, J. Biol. Chem. 241 (1966), 2280-2285). Furthermore, mutants of potato and pea have been described which synthesize starches with a high amylose content (70% in maize or up to 50% in pea). These mutants have so far not been characterized on the molecular level and therefore do not allow for the production of corresponding mutants in other starch-storing plants.

Alternatively, plants synthesizing starch with altered properties may be produced by means of recombinant DNA techniques. In various cases, for example, the recombinant modification of potato plants aiming at altering the starch synthesized in these plants has been described (e.g. WO 92/11376; WO 92/14827). However, in order to make use of recombinant DNA techniques, DNA sequences are required the gene products of which influence starch synthesis, starch modification or starch degradation.

Therefore, the problem underlying the present invention is to provide nucleic acid molecules and methods which allow for the alteration of plants in such a way, that they synthesize a starch which differs from starch naturally synthesized in plants with respect to its physical and/or chemical properties, in particular a highly amylose-containing starch, and is therefore more suitable for general and/or particular uses.

This problem is solved by the provision of the embodiments described in the claims.

Therefore, the present invention relates to nucleic acid molecules encoding a protein with the amino acid sequence indicated in Seq ID No. 2. Such proteins are present in the plastids of plant cells, bound to starch granules as well as in free, i.e. soluble form. During the expression of E.coli, the enzyme activity of such proteins leads to an increased phosphorylation of the glycogen synthesized within the cells. The molecular weight of these proteins lies within the range of 140-160 kD if it is assessed by means of a SDS gel electrophoresis.

The present invention further relates to nucleic acid molecules comprising a sequence with the nucleotide sequence indicated in Seq ID No. 1, particularly the coding region indicated in Seq ID No. 1.

Nucleic acid molecules encoding a protein from potato, which in the plastids of the cells is partly granule-bound, and hybridizing to the above-mentioned nucleic acid molecules of the invention or their complementary strand are also the subject matter of the present invention. In this context the term “hybridization” signifies hybridization under conventional hybridizing conditions, preferably under stringent conditions as described for example in Sambrook et al., Molecular Cloning, A Laboratory Manual, 2^(nd) Edition (1989) Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.). These nucleic acid molecules hybridizing with the nucleic acid molecules of the invention may principally be derived from any desired organism (i.e. prokaryotes or eukaryotes, in particular from bacteria, fungi, alga, plants or animal organisms) comprising such nucleic acid molecules. They are preferably derived from monocotyledonous or dicotyledonous plants, particularly from useful plants, and particularly preferred from starch-storing plants.

Nucleic acid molecules hybridizing to the molecules according to the invention may be isolated e.g. from genomic or from cDNA libraries of various organisms.

Thereby, the identification and isolation of such nucleic acid molecules may take place by using the molecules according to the invention or parts of these molecules or, as the case may be, the reverse complement strands of these molecules, e.g. by hybridization according to standard methods (see e.g. Sambrook et al., 1989, Molecular Cloning, A Laboratory Manual, 2nd Edition, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.).

As a probe for hybridization e.g. nucleic acid molecules may be used which exactly or basically contain the nucleotide sequence indicated under Seq ID No. 1 or parts thereof. The DNA fragments used as hybridization probe may also be synthetic DNA fragments which were produced by means of the conventional DNA synthesizing methods and the sequence of which is basically identical with that of a nucleic acid molecule of the invention. After identifying and isolating genes hybridizing to the nucleic acid sequences according to the invention, the sequence has to be determined and the properties of the proteins encoded by this sequence have to be analyzed.

Furthermore, the present invention relates to nucleic acid molecules the sequences of which, compared to the sequences of the above-mentioned molecules, are degenerated due to the genetic code and which encode a protein which in the plastids of plant cells is partly granule-bound.

Fragments, derivatives and allelic variants of the above-mentioned nucleic acid molecules, which encode the above-mentioned protein are also the subject matter of the present invention. Thereby, fragments are described as: parts of the nucleic acid molecules which are long enough in order to encode the above-described protein. In this context, the term derivative signifies that the sequences of these molecules differ from the sequences of the above-mentioned nucleic acid molecules at one or more positions and exhibit a high degree of homology to the sequences of these molecules. Hereby, homology means a sequence identity of at least 40%, in particular an identity of at least 60%, preferably of more than 80% and still more preferably a sequence identity of more than 90%. The deviations occurring when comparing with the above-described nucleic acid molecules might have been caused by deletion, substitution, insertion or recombination.

Moreover, homology means that functional and/or structural equivalence exists between the respective nucleic acid molecules or the proteins they encode. The nucleic acid molecules, which are homologous to the above-described nucleic acid molecules and represent derivatives of these molecules, are generally variations of these nucleic acid molecules, that constitute modifications which exert the same biological function. These variations may be naturally occurring variations, for example sequences from different organisms, or mutations, whereby these mutations may have occurred naturally or they may have been introduced deliberately. Moreover the variations may be synthetically produced sequences.

The allelic variants may be naturally occurring as well as synthetically produced variants or variants produced by recombinant DNA techniques.

The proteins encoded by the various variants of the nucleic acid molecules according to the invention exhibit certain common characteristics. Enzyme activity, molecular weight, immunologic reactivity, conformation etc. may belong to these characteristics as well as physical properties such as the mobility in gel electrophoresis, chromatographic characteristics, sedimentation coefficients, solubility, spectroscopic properties, stability, pH-optimum, temperature-optimum etc.

The nucleic acid molecules of the invention may principally be derived from any organism expressing the described proteins. They are preferably derived from plants, in particular from starch-synthesizing or starch-storing plants. Cereals (such as barley, rye, oats, wheat etc.), maize, rice, pea, cassava, potato etc. are particularly preferred. They can also be produced by means of synthesis methods known to the skilled person.

The nucleic acid molecules of the invention may be DNA molecules, such as cDNA or genomic DNA, as well as RNA molecules.

Furthermore, the invention relates to vectors, especially plasmids, cosmids, viruses, bacteriophages and other vectors common in genetic engineering, which contain the above-mentioned nucleic acid molecules of the invention.

In a preferred embodiment the nucleic acid molecules contained in the vectors are linked to regulatory elements that ensure the transcription and synthesis of a translatable RNA in prokaryotic and eukaryotic cells.

In a further embodiment the invention relates to host cells, in particular prokaryotic or eukaryotic cells, which have been transformed and/or recombinantly manipulated by an above-mentioned nucleic acid molecule of the invention or by a vector of the invention, as well as cells derived from such cells and containing a nucleic acid molecule of the invention or a vector of the invention. This is preferably a bacterial cell or a plant cell.

It was now found that the protein encoded by the nucleic acid molecules of the invention influences the starch synthesis or modification and that changes in the amount of the protein in plant cells lead to changes in the starch metabolism of the plant, especially to the synthesis of starch with modified physical and chemical properties.

By providing the nucleic acid molecules of the invention it is possible to produce plants by means of recombinant DNA techniques synthesizing a modified starch which differs from the starch synthesized in wildtype plants with respect to its structure and its physical and chemical properties. For this purpose, the nucleic acid molecules of the invention are linked to regulatory elements, which ensure the transcription and translation in plant cells, and they are introduced into the plant cells.

Therefore, the present invention also relates to transgenic plant cells containing a nucleic acid molecule of the invention whereby the same is linked to regulatory elements which ensure the transcription in plant cells. The regulatory elements are preferably heterologous with respect to the nucleic acid molecule.

By means of methods known to the skilled person the transgenic plant cells can be regenerated to whole plants. The plants obtainable by regenerating the transgenic plant cells of the invention are also the subject-matter of the present invention. A further subject-matter of the invention are plants which contain the above-described transgenic plant cells. The transgenic plants may in principle be plants of any desired species, i.e. they may be monocotyledonous as well as dicotyledonous plants. These are preferably useful plants, in particular starch-storing plants such as cereals (rye, barley, oats, wheat etc.), rice, maize, peas, cassava and potatoes.

Due to the expression or the additional expression of a nucleic acid molecule of the invention, the transgenic plant cells and plants of the invention synthesize a starch which is modified when compared to starch from wildtype-plants, i.e. non-transformed plants, particularly with respect to the viscosity of aqueous solutions of this starch and/or to the phosphate content. The latter is generally increased in the starch of transgenic plant cells or plants, this altering the physical properties of the starch.

Therefore, the starch obtainable from the transgenic plant cells and plants of the invention is also the subject-matter of the present invention.

A further subject-matter of the present invention is a method for the production of a protein which is present in plant cells in granule-bound form as well as in soluble from, in which host cells of the invention are cultivated under conditions that allow for the expression of the protein and in which the protein is then isolated from the cultivated cells and/or the culture medium.

Furthermore, the invention relates to proteins encoded by the nucleic acid molecules of the invention as well as to proteins obtainable by the above-described method. These are preferably proteins encoded by nuclear genes and which are localized in the plastids. In the plastids these enzymes are present in granule-bound as well as in free form. In an SDS gel electrophoresis, the respective proteins from Solanum tuberosum exhibit a molecular weight of 140-160 kD and, during the expression of E.coli, lead to an increased phosphorylation of the glycogen synthesized within the cells.

A further subject-matter of the invention are antibodies which specifically recognize a protein of the invention. These may be monoclonal as well as polyclonal antibodies.

Furthermore, the present invention relates to nucleic acid molecules specifically hybridizing with a nucleic acid molecule of the invention and exhibiting a length of at least 15 nucleotides. In this context specifically hybridizing signifies that under conventional hybridization conditions, preferably under stringent conditions, cross-hybridization with sequences encoding other proteins does not significantly occur. Such nucleic acid molecules preferably have a length of at least 20, more preferably a length of at least 50 and most preferably a length of at least 100 nucleotides. Such molecules can be used, for example, as PCR primers, as hybridization probes or as DNA molecules which encode antisense RNA.

Furthermore, it was found that it is possible to influence the properties of the starch synthesized in plant cells by reducing the amount of proteins encoded by the nucleic acid molecules according to the invention in the cells. This reduction may be effected, for example, by means of antisense expression of the nucleic acid molecules of the invention, expression of suitable ribozymes or cosuppression.

Therefore, DNA molecules encoding an antisense RNA which is complementary to transcripts of a DNA molecule of the invention are also the subject-matter of the present invention, as well as these antisense molecules. Thereby, complementary does not signify that the encoded RNA has to be 100% complementary. A low degree of complementarity is sufficient, as long as it is high enough in order to inhibit the expression of a protein of the invention upon expression in plant cells. The transcribed RNA is preferably at least 90% and most preferably at least 95% complementary to the transcript of the nucleic acid molecule of the invention. In order to cause an antisense-effect during the transcription in plant cells such DNA molecules have a length of at least 15 bp, preferably a length of more than 100 bp and most preferably a length of more than 500 bp, however, usually less than 5000 bp, preferably shorter than 2500 bp.

The invention further relates to DNA molecules which, during expression in plant cells, lead to the synthesis of an RNA which in the plant cells due to a cosupression-effect reduces the expression of the nucleic acid molecules of the invention encoding the described protein. The principle of the cosupression as well as the production of corresponding DNA sequences is precisely described, for example, in WO 90/12084. Such DNA molecules preferably encode a RNA having a high degree of homology to transcripts of the nucleic acid molecules of the invention. It is, however, not absolutely necessary that the coding RNA is translatable into a protein.

In a further embodiment the present invention relates to DNA molecules encoding an RNA molecule with ribozyme activity which specifically cleaves transcripts of a DNA molecule of the invention as well as these encoded RNA molecules.

Ribozymes are catalytically active RNA molecules capable of cleaving RNA molecules and specific target sequences. By means of recombinant DNA techniques it is possible to alter the specificity of ribozymes. There are various classes of ribozymes. For practical applications aiming at the specific cleavage of the transcript of a certain gene, use is preferably made of representatives of two different groups of ribozymes. The first group is made up of ribozymes which belong to the group I intron ribozyme type. The second group consists of ribozymes which as a characteristic structural feature exhibit the so-called “hammerhead” motif. The specific recognition of the target RNA molecule may be modified by altering the sequences flanking this motif. By base pairing with sequences in the target molecule these sequences determine the position at which the catalytic reaction and therefore the cleavage of the target molecule takes place. Since the sequence requirements for an efficient cleavage are extremely low, it is in principle possible to develop specific ribozymes for practically each desired RNA molecule.

In order to produce DNA molecules encoding a ribozyme which specifically cleaves transcripts of a DNA molecule of the invention, for example a DNA sequence encoding a catalytic domain of a ribozyme is bilaterally linked with DNA sequences which are homologous to sequences of the target enzyme.

Sequences encoding the catalytic domain may for example be the catalytic domain of the satellite DNA of the SCMo virus (Davies et al., Virology 177 (1990), 216-224) or that of the satellite DNA of the TobR virus (Steinecke et al., EMBO J. 11 (1992), 1525-1530; Haseloff and Gerlach, Nature 334 (1988), 585-591). The DNA sequences flanking the catalytic domain are preferably derived from the above-described DNA molecules of the invention.

In a further embodiment the present invention relates to vectors containing the above-described DNA molecules, in particular those in which the described DNA molecules are linked with regulatory elements ensuring the transcription in plant cells.

Furthermore, the present invention relates to host cells containing the described DNA molecules or vectors. The host cell may be a prokaryotic cell, such as a bacterial cell, or a eukaryotic cell. The eucaryotic host cells are preferably plant cells.

Furthermore, the invention relates to transgenic plant cells containing an above-described DNA molecule encoding an antisense-RNA, a ribozyme or an RNA which leads to a cosuppression effect, whereby the DNA molecule is linked to DNA elements ensuring the transcription in plant cells. These transgenic plant cells may be regenerated to whole plants according to well-known techniques. Thus, the invention also relates to plants which may be obtained through regeneration from the described transgenic plant cells, as well as to plants containing the described transgenic plant cells. The transgenic plants themselves may be plants of any desired plant species, preferably useful plants, particularly starch-storing ones, as indicated above.

Due to the expression of the described DNA molecules encoding antisense RNA, a ribozyme or a cosupression RNA in the transgenic plant cells the amount of proteins encoded by the DNA molecules of the invention which are present in the cells in endogenic form, is reduced. Surprisingly, this reduction leads to a drastic change of the physical and chemical properties of the starch synthesized in the plant cells, in particular with respect to the viscous properties of the aqueous solutions of this starch, to the phosphate content as well as to the release of reducing sugars in the storage of the plant cells or plant parts at low temperatures. The properties of the starch synthesized in the transgenic plant cells is explicitely described below.

Thus, the starch obtainable from the described transgenic plant cells and plants is also the subject matter of the present invention.

Furthermore, the invention relates to the antisense RNA molecules encoded by the described DNA molecules, as well as to RNA molecules with ribozyme activity and RNA molecules which lead to a cosupression effect which are obtainable, for example, by means of transcription.

A further subject-matter of the invention is a method for the production of transgenic plant cells, which in comparison to non-transformed cells synthesize a modified starch. In this method the amount of proteins encoded by the DNA molecules of the invention, which are present in the cells in endogenic form, is reduced in the plant cells.

In a preferred embodiment this reduction is effected by means of an antisense effect. For this purpose the DNA molecules of the invention or parts thereof are linked in antisense orientation with a promoter ensuring the transcription in plant cells and possibly with a termination signal ensuring the termination of the transcription as well as the polyadenylation of the transcript. In order to ensure an efficient antisense effect in the plant cells the synthesized antisense RNA should exhibit a minimum length of 15 nucleotides, preferably of at least 100 nucleotides and most preferably of more than 500 nucleotides. Furthermore, the DNA sequence encoding the antisense RNA should be homologous with respect to the plant species to be transformed. However, DNA sequences exhibiting a high degree of homology to DNA sequences which are present in the cells in endogenic form may also be used, preferably with an homology of more than 90% and most preferably with an homology of more than 95%.

In a further embodiment the reduction of the amount of proteins encoded by the DNA molecules of the invention is effected by a ribozyme effect. The basic effect of ribozymes as well as the construction of DNA molecules encoding such RNA molecules have already been described above. In order to express an RNA with ribozyme activity in transgenic cells the above described DNA molecules encoding a ribozyme are linked with DNA elements which ensure the transcription in plant cells, particularly with a promoter and a termination signal. The ribozymes synthesized in the plant cells lead to the cleavage of transcripts of DNA molecules of the invention which are present in the plant cells in endogenic form.

A further possibility in order to reduce the amount of proteins encoded by the nucleic acid molecules of the invention is cosupression. Therefore, the plant cells obtainable by the method of the invention are a further subject matter. These plant cells are characterized in that their amount of proteins encoded by the DNA molecules of the invention is reduced and that in comparison to wildtype cells they synthesize a modified starch.

Furthermore, the invention relates to plants obtainable by regeneration of the described plant cells as well as to plants containing the described cells of the invention.

The starch obtainable from the described plant cells and plants is also the subject-matter of the present invention. This starch differs from starch obtained from non-transformed cells or plants in its physical and/or chemical properties. When compared to starch from wildtype plants, the starch exhibits a reduced phosphate content. Makeover, the aqueous solutions of this starch exhibit modified viscous properties.

In a preferred embodiment the phosphate content of the described starch is reduced by at least 50%, more preferably by at least 75% and in a particularly preferred embodiment by more than 80% in comparison to starch derived from wildtype plants.

The modified viscosity of the aqueous solution of this starch is its most advantageous feature.

A well-established test for determining the viscosity is the so-called Brabender test. This test is carried out by using an appliance which is for example known as viscograph E. This equipment is produced and sold, among others, by Brabender fOHG Duisburg (Germany).

The test basically consists in first heating starch in the presence of water in order to assess when hydratization and the swelling of the starch granules takes place. This process which is also named gelatinization or pastification is based on the dissolving the hydrogen bonds and involves a measurable increase of the viscosity in the starch suspension. While further heating after gelatinization leads to the complete dissolving of the starch particles and to a decrease of viscosity, the immediate cooling after gelatinization typically leads to a increase in the viscosity (see FIG. 3). The result of the Brabender test is a graph which shows the viscosity depending on time, whereby at first the solution is heated to above the gelatinization temperature and then cooled.

The analysis of the Brabender graph is generally directed to determining the pastification temperature, the maximum viscosity during heating, the increase in viscosity during cooling, as well as the viscosity after cooling. These parameters are important characteristics when it comes to the quality of a starch and the possibilty to use it for various purposes.

The starch which may for example be isolated from potato plants in which the amount of proteins of the invention within the cells was reduced by means of an antisense effect, showed characteristics strongly deviating from the characteristics of starch isolated from wildtype plants. Compared with these it only shows a low increase in viscosity during heating, a low maximum viscosity as well as a stronger increase in viscosity during cooling (see FIGS. 3, 4 and 5).

In a preferred embodiment the invention relates to starch, the aqueous solutions of which exhibit the characteristic viscous properties depicted in FIGS. 4 or 5. Particularly under the conditions mentioned in Example 8a for determining the viscosity with the help of a Brabender viscosimeter, the modified starch, when compared to wildtype plants, exhibits the characteristic of only a low increase in viscosity when heating the solution. This offers the opportunity of using the starch for the production of highly-concentrated glues.

Moreover, after reaching maximum viscosity, there is only a low decrease in viscosity in the case of the modified starch. On the other hand the viscosity increases strongly on cooling; thus, the viscosity of modified starch is higher than the viscosity of starch from wildtype plants.

By reducing the amount of proteins of the invention in transgenic plant cells it is furthermore possible to produce a starch which has the effect that when plant parts containing this starch are stored at low temperatures, in particular at 4-8° C., less reducing sugars are released than is the case which starch from non-transformed cells. This property is particularly advantageous, for example, for providing potatoes which during storage at low temperatures release less reducing sugars and thus exhibit a reduced cold sweetening. Such potatoes are particularly suitable for producing French fries, crisps or similar products since undesirable browning-reactions (Maillard reactions) are avoided or at least strongly reduced during use.

In a particularly preferred embodiment of the present invention not only the synthesis of a protein of the invention is reduced in the transformed plant cells, but moreover also the synthesis of at least one further enzyme involved in starch synthesis and/or modification. In this context, for example, starch granule-bound starch synthases or branching enzymes are preferred. Surprisingly, it was found that potato plants in which the synthesis of the proteins of the invention as well as of the branching enzyme is reduced due to an antisense effect synthesize a starch which in its properties strongly deviates from starch of wildtype plants.

When compared to wildtype starch, the aqueous solutions of this modified starch show almost no increase in viscosity during heating or cooling (cf. FIG. 6).

Furthermore, a microscopical analysis of the starch granules before and after heating clearly shows that, when compared to wildtype plants, the starch granules of plants modified in such a way are not open but remain basically unchanged in their structure. Thus, this is a starch which is resistent to the heating process. If the amylose content of this starch is determined by means of the method described in the Examples, amylose contents of more than 50%, preferably of more than 60% and most preferably of more than 70% are measured for this starch. The aqueous solutions of the starch isolated from this plants preferably show the characteristic viscous properties depicted in FIG. 6.

Such a highly amylose-containing starch of the invention offers a number of advantages for various uses when compared to wildtype plants. Thus, highly amylose-containing starches have a high potential for the use in foils and films. The foils and films produced on the basis of highly amylose-containing starches, which may be used in wide areas of the packaging industry, have the essential advantage of being biodegradable. Apart from this use which is basically covered by classical, petrochemically produced polymers, amylose has further unique fields of application which are caused by the amylose's property to form helices. The helix formed by the amylose is internally hydrophobic and externally hydrophilic. Due to this, amylose may be used for the complexation and molecular encapsulation of low molecular or also of high molecular substances. Examples therefore are:

-   -   the molecular encapsulation of vitamines and substances for the         protection against oxidation, volatilization, thermal         degradation or the transition into an aqueous environment;     -   the molecular encapsulation of aromatic substances for         increasing the solubility;     -   the molecular encapsulation of fertilizers/pesticides for         stabilization and controlled release;     -   the molecular encapsulation of medical substances for         stabilizing the dosage-control and for the controlled release of         retarding preparations.

Another important property of amylose is the fact that it is a chiral molecule. Due to the chirality it may preferably be used after immobilization, e.g. on a column for separating enantiomers.

Furthermore, it was surprisingly found that starch which may be isolated from potato plants in which the amount of proteins of the invention in the cells was reduced due to an antisense effect, in combination with a reduction of the proteins exhibiting the enzymatic activity of a starch granule-bound starch synthase of the isotype I (GBSSI) exhibits characteristics which strongly deviate from the characteristics of starch which may be isolated from wildtype plants. When compared to starch from wildtype plants, the aqueous solutions of this starch only show a low increase in viscosity during heating, a low maximum viscosity as well as almost no increase in viscosity during cooling (cf. FIG. 7). If the amylose/amylopectin ratio of this starch is determined, this starch is characterized in that almost no amylose can be measured. The amylose content of this starch is preferably below 5% and most preferably below 2%. The starch of the invention furthermore differs from the known starch which may be produced in transgenic potato plants by inhibiting the GBSSI gene solely by means of recombinant DNA techniques. Thus, this starch shows a strong increase in viscosity during heating. The aqueous solutions of the starch of the invention preferably show the characteristic viscous properties depicted in FIG. 7. Particularly under the conditions for determining the viscosity by means of a Rapid Visco Analyser described in Example 13, the modified starch has the characteristic of only exhibiting a low viscosity increase during heating when compared to wildtype starch, but also when compared to waxy starch. This offers the opportunity to use the starch of the invention for the production of highly-concentrated glues. The modified starch furthermore has the property that there is only a low decrease of viscosity after reaching the maximum viscosity, as well as almost no increase in viscosity during cooling.

Possibilities in order to reduce the activity of a branching enzyme in plant cells were already described, for example in WO 92/14827 and WO 95/26407. The reduction of the activity of a starch granule-bound starch synthase of the isotype I (GBSSI) may be carried out by using methods known to the skilled person, e.g. by means of an antisense effect. DNA sequences encoding a GBSSI from potatoe are for example known from Hegersberg (dissertation (1988) University of Cologne), Visser et al. (Plant Sci. 64 (1989), 185-192) or van der Leiy et al. (Mol. Gen. Genet. 228 (1991), 240-248).

The method of the invention may in principle be used for any kind of plant species. Monocotyledonous and dicotyledonous plants are of interest, in particular useful plants and preferably starch-storing plants such as cereals (rye, barley, oats, wheat etc.), rice, maize, pea, cassava and potatoes.

Within the framework of the present invention the term “regulatory DNA elements ensuring the transcription in plant cells” are DNA regions which allow for the initiation or the termination of transcription in plant cells. DNA regions ensuring the initiation of transcription are in particular promoters.

For the expression of the various above-described DNA molecules of the invention in plants any promoter functioning in plant cells may be used. The promoter may be homologous or heterologous with respect to the used plant species. Use may, for example, be made of the 35S promoter of the cauliflower mosaic virus (Odell et al., Nature 313 (1985), 810-812) which ensures a constitutive expression in all plant tissues and also of the promoter construct described in WO/9401571. However, use may also be made of promoters which lead to an expression of subsequent sequences only at a point of time determined by exogenous factors (such as in WO/9307279) or in a particular tissue of the plant (see e.g. Stockhaus et al., EMBO J. 8 (1989), 2245-2251). Promoters which are active in the starch-storing parts of the plant to be transformed are preferably used. In the case of potato these parts are the potato seeds, in the case of potatoes the tubers. In order to transform potatoes the tuber-specific B33-promoter (Rocha-Sosa et al., EMBO J. 8 (1989), 23-29) may be used particularly, but not exclusively.

Apart from promoters, DNA regions initiating transcription may also contain DNA sequences ensuring a further increase of transcription, such as the so-called enhancer-elements. Furthermore, the term “regulatory DNA elements” may also comprise termination signals which serve to correctly end the transcription and to add a poly-A-tail to the transcript which is believed to stabilize the transcripts. Such elements are described in the literature and can be exchanged as desired. Examples for such termination sequences are the 3′-nontranslatable regions comprising the polyadenylation signal of the nopaline synthase gene (NOS gene) or the octopine synthase gene (Gielen et al., EMBO J. 8 (1989), 23-29) from agrobacteria, or the 3′-nontranslatable regions of the genes of the storage proteins from soy bean as well as the genes of the small subunit of ribulose-1,5-biphosphate-carboxylase (ssRUBISCO).

The introduction of the DNA molecules of the invention into plant cells is preferably carried out using plasmids. Plasmids ensuring a stable integration of the DNA into the plant genome are preferred.

In the examples of the present invention use is made of the binary vector pBinAR (Höfgen and Willmitzer, Plant Sci. 66 (1990), 221-230). This vector is a derivative of the binary vector pBin19 (Bevan, Nucl. Acids Res. 12 (1984), 8711-8721), which may commercially be obtained (Clontech Laboratories, Inc. USA).

However, use may be made of any other plant transformation vector which can be inserted into a expression cassette and which ensures the integration of the expression cassette into the plant genome.

In order to prepare the introduction of foreign genes in higher plants a large number of cloning vectors are at disposal, containing a replication signal for E.coli and a marker gene for the selection of transformed bacterial cells. Examples for such vectors are pBR322, pUC series, M13mp series, pACYC184 etc. The desired sequence may be integrated into the vector at a suitable restriction site. The obtained plasmid is used for the transformation of E.coli cells. Transformed E.coli cells are cultivated in a suitable medium and subsequently harvested and lysed. The plasmid is recovered by means of standard methods. As an analyzing method for the characterization of the obtained plasmid DNA use is generally made of restriction analysis and sequence analysis. After each manipulation the plasmid DNA may be cleaved and the obtained DNA fragments may be linked to other DNA sequences.

In order to introduce DNA into plant host cells a wide range of techniques are at disposal. These techniques comprise the transformation of plant cells with T-DNA by using Agrobacterium tumefaciens or Agrobacterium rhizogenes as transformation medium, the fusion of protoplasts, the injection and the electroporation of DNA, the introduction of DNA by means of the biolistic method as well as further possibilities.

In the case of injection and electroporation of DNA into plant cells, there are no special demands made to the plasmids used. Simple plasmids such as pUC derivatives may be used. However, in case that whole plants are to be regenerated from cells transformed in such a way, a selectable marker gene should be present.

Depending on the method of introducing desired genes into the plant cell, further DNA sequences may be necessary. If the Ti- or Ri-plasmid is used e.g. for the transformation of the plant cell, at least the right border, more frequently, however, the right and left border of the Ti- and Ri-plasmid T-DNA has to be connected to the foreign gene to be introduced as a flanking region.

If Agrobacteria are used for transformation, the DNA which is to be introduced must be cloned into special plasmids, namely either into an intermediate vector or into a binary vector. Due to sequences homologous to the sequences within the T-DNA, the intermediate vectors may be integrated into the Ti- or Ri-plasmid of the Agrobacterium due to homologous recombination. This also contains the vir-region necessary for the transfer of the T-DNA. Intermediate vectors cannot replicate in Agrobacteria. By means of a helper plasmid the intermediate vector may be transferred to Agrobacterium tumefaciens (conjugation). Binary vectors may replicate in E.coli as well as in Agrobacteria. They contain a selectable marker gene as well as a linker or polylinker which is framed by the right and the left T-DNA border region. They may be transformed directly into the Agrobacteria (Holsters et al. Mol. Gen. Genet. 163 (1978), 181-187). The plasmids used for the transformation of the Agrobacteria further comprise a selectable marker gene, such as the NPT II gene which allows for selecting transformed bacteria. The Agrobacterium acting as host cell should contain a plasmid carrying a vir-region. The vir-region is necessary for the transfer of the T-DNA into the plant cell. Additional T-DNA may be present. The Agrobacterium transformed in such a way is used for the transformation of plant cells.

The use of T-DNA for the transformation of plant cells was investigated intensely and described sufficiently in EP 120 516; Hoekema, In: The Binary Plant Vector System Offsetdrukkerij Kanters B. V., Alblasserdam (1985), Chapter V; Fraley et al., Crit. Rev. Plant. Sci., 4, 1-46 and An et al. EMBO J. 4 (1985), 277-287. Some binary vectors may already be obtained commercially, such as pBIN19 (Clontech Laboratories, Inc., USA).

For transferring the DNA into the plant cells, plant explants may suitably be co-cultivated with Agrobacterium tumefaciens or Agrobacterium rhizogenes. From the infected plant material (e.g. pieces of leaves, stem segments, roots, but also protoplasts or suspension-cultivated plant cells) whole plants may then be regenerated in a suitable medium which may contain antibiotics or biozides for the selection of transformed cells. The plants obtained in such a way may then be examined as to whether the introduced DNA is present or not.

Once the introduced DNA has been integrated in the genome of the plant cell, it usually continues to be stable there and also remains within the descendants of the originally transformed cell. It usually contains a selectable marker which confers resistance against birzides or against an antibiotic such as kanamycin, G 418, bleomycin, hygromycin or phosphinotricine etc. to the transformed plant cells. The individually selected marker should therefore allow for a selection of transformed cells against cells lacking the introduced DNA.

The transformed cells grow in the usual way within the plant (see also McCormick et al., Plant Cell Reports 5 (1986), 81-84). The resulting plants can be cultivated in the usual way and cross-bred with plants having the same transformed genetic heritage or another genetic heritage. The resulting hybrid individuals have the corresponding phenotypic properties.

Two or more generations should be grown in order to ensure whether the phenotypic feature is kept stably and whether it is transferred. Furthermore, seeds should be harvested in order to ensure that the corresponding phenotype or other properties will remain.

Due to its properties the starch obtained from the plant cells or from the plants of the invention is not only suitable for the specific purposes already mentioned herein, but also for various industrial uses.

Basically, starch can be subdivided into two major fields. One field comprises the hydrolysis products of starch and the so-called native starches. The hydrolysis products essentially comprise glucose and glucans components obtained by enzymatic or chemical processes. They can be used for further processes, such as fermentation and chemical modifications. In this context, it might be of importance that the hydrolysis process can be carried out simply and inexpensively. Currently, it is carried out substantially enzymatically using amyloglucosidase. It is thinkable that costs might be reduced by using lower amounts of enzymes for hydrolysis due to changes in the starch structure, e.g. increasing the surface of the grain, improved digestibility due to less branching or a steric structure, which limits the accessibility for the used enzymes.

The use of the so-called native starch which is used because of its polymer structure can be subdivided into two further areas:

(a) Use in Foodstuffs

Starch is a classic additive for various foodstuffs, in which it essentially serves the purpose of binding aqueous additives and/or causes an increased viscosity or an increased gel formation. Important characteristic properties are flowing and sorption behavior, swelling and pastification temperature, viscosity and thickening performance, solubility of the starch, transparency and paste structure, heat, shear and acid resistance, tendency to retrogradation, capability of film formation, resistance to freezing/thawing, digestibility as well as the capability of complex formation with e.g. inorganic or organic ions.

(b) Use in Non-Foodstuffs

The other major field of application is the use of starch as an adjuvant in various production processes or as an additive in technical products. The major fields of application for the use of starch as an adjuvant are, first of all, the paper and cardboard industry. In this field, the starch is mainly used for retention (holding back solids), for sizing filler and fine particles, as solidifying substance and for dehydration. In addition, the advantageous properties of starch with regard to stiffness, hardness, sound, grip, gloss, smoothness, tear strength as well as the surfaces are utilized.

Within the paper production process, a differentiation can be made between four fields of application, namely surface, coating, mass and spraying.

The requirements on starch with regard to surface treatment are essentially a high degree of brightness, corresponding viscosity, high viscosity stability, good film formation as well as low formation of dust. When used in coating the solid content, a corresponding viscosity, a high capability to bind as well as a high pigment affinity play an important role. As an additive to the mass rapid, uniform, loss-free dispersion, high mechanical stability and complete retention in the paper pulp are of importance. When using the starch in spraying, corresponding content of solids, high viscosity as well as high capability to bind are also significant.

A major field of application is, for instance, in the adhesive industry, where the fields of application are subdivided into four areas: the use as pure starch glue, the use in starch glues prepared with special chemicals, the use of starch as an additive to synthetic resins and polymer dispersions as well as the use of starches as extenders for synthetic adhesives. 90% of all starch-based adhesives are used in the production of corrugated board, paper sacks and bags, composite materials for paper and aluminum, boxes and wetting glue for envelopes, stamps, etc.

Another possible use as adjuvant and additive is in the production of textiles and textile care products. Within the textile industry, a differentiation can be made between the following four fields of application: the use of starch as a sizing agent, i.e. as an adjuvant for smoothing and strengthening the burring behavior for the protection against tensile forces active in weaving as well as for the increase of wear resistance during weaving, as an agent for textile improvement mainly after quality-deteriorating pretreatments, such as bleaching, dying, etc., as thickener in the production of dye pastes for the prevention of dye diffusion and as an additive for warping agents for sewing yarns.

Furthermore, starch may be used as an additive in building materials. One example is the production of gypsum plaster boards, in which the starch mixed in the thin plaster pastifies with the water, diffuses at the surface of the gypsum board and thus binds the cardboard to the board. Other fields of application are admixing it to plaster and mineral fibers. In ready-mixed concrete, starch may be used for the deceleration of the sizing process.

Furthermore, the starch is advantageous for the production of means for ground stabilization used for the temporary protection of ground particles against water in artificial earth shifting. According to state-of-the-art knowledge, combination products consisting of starch and polymer emulsions can be considered to have the same erosion- and encrustation-reducing effect as the products used so far; however, they are considerably less expensive.

Another field of application is the use of starch in plant protectives for the modification of the specific properties of these preparations. For instance, starches are used for improving the wetting of plant protectives and fertilizers, for the dosed release of the active ingredients, for the conversion of liquid, volatile and/or odorous active ingredients into microcristalline, stable, deformable substances, for mixing incompatible compositions and for the prolongation of the duration of the effect due to a reduced disintegration.

Starch may also be used in the fields of drugs, medicine and in the cosmetics industry. In the pharmaceutical industry, the starch may be used as a binder for tablets or for the dilution of the binder in capsules. Furthermore, starch is suitable as disintegrant for tablets since, upon swallowing, it absorbs fluid and after a short time it swells so much that the active ingredient is released. For qualitative reasons, medicinal flowance and dusting powders are further fields of application. In the field of cosmetics, the starch may for example be used as a carrier of powder additives, such as scents and salicylic acid. A relatively extensive field of application for the starch is toothpaste.

The use of starch as an additive in coal and briquettes is also thinkable. By adding starch, coal can be quantitatively agglomerated and/or briquetted in high quality, thus preventing premature disintegration of the briquettes. Barbecue coal contains between 4 and 6%, added starch, calorated coal between 0.1 and 0.5%. Furthermore, the starch is suitable as a binding agent since adding it to coal and briquette can considerably reduce the emission of toxic substances.

Furthermore, the starch may be used as a flocculant in the processing of ore and coal slurry.

Another field of application is the use as an additive to process materials in casting. For various casting processes cores produced from sands mixed with binding agents are needed. Nowadays, the most commonly used binding agent is bentonite mixed with modified starches, mostly swelling starches.

The purpose of adding starch is increased flow resistance as well as improved binding strength. Moreover, swelling starches may fulfill more prerequisites for the production process, such as dispersability in cold water, rehydratisability, good mixability in sand and high capability of binding water.

In the rubber industry starch may be used for improving the technical and optical quality. Reasons for this are improved surface gloss, grip and appearance. For this purpose, the starch is dispersed on the sticky rubberized surfaces of rubber substances before the cold vulcanization. It may also be used for improving the printability of rubber.

Another field of application for the modified starch is the production of leather substitutes.

In the plastics market the following fields of application are emerging: the integration of products derived from starch into the processing process (starch is only a filler, there is no direct bond between synthetic polymer and starch) or, alternatively, the integration of products derived from starch into the production of polymers (starch and polymer form a stable bond).

The use of the starch as a pure filler cannot compete with other substances such as talcum. This situation is different when the specific starch properties become effective and the property profile of the end products is thus clearly changed. One example is the use of starch products in the processing of thermoplastic materials, such as polyethylene. Thereby, starch and the synthetic polymer are combined in a ratio of 1:1 by means of coexpression to form a ‘master batch’, from which various products are produced by means of common techniques using granulated polyethylene. The integration of starch in polyethylene films may cause an increased substance permeability in hollow bodies, improved water vapor permeability, improved antistatic behavior, improved anti-block behavior as well as improved printability with aqueous dyes.

Another possibility is the use of the starch in polyurethane foams. Due to the adaptation of starch derivatives as well as due to the optimization of processing techniques, it is possible to specifically control the reaction between synthetic polymers and the starch's hydroxy groups. The results are polyurethane films having the following property profiles due to the use of starch: a reduced coefficient of thermal expansion, decreased shrinking behavior, improved pressure/tension behavior, increased water vapor permeability without a change in water acceptance, reduced flammability and cracking density, no drop off of combustible parts, no halides and reduced aging. Disadvantages that presently still exist are reduced pressure and impact strength.

Product development of film is not the only option. Also solid plastics products, such as pots, plates and bowls can be produced by means of a starch content of more than 50%. Furthermore, the starch/polymer mixtures offer the advantage that they are much easier biodegradable.

Furthermore, due to their extreme capability to bind water, starch graft polymers have gained utmost importance. These are products having a backbone of starch and a side lattice of a synthetic monomer grafted on according to the principle of radical chain mechanism. The starch graft polymers available nowadays are characterized by an improved binding and retaining capability of up to 1000 g water per g starch at a high viscosity. These super absorbers are used mainly in the hygiene field, e.g. in products such as diapers and sheets, as well as in the agricultural sector, e.g. in seed pellets.

What is decisive for the use of the new starch modified by recombinant DNA techniques are, on the one hand, structure, water content, protein content, lipid content, fiber content, ashes/phosphate content, amylose/amylopectin ratio, distribution of the relative molar mass, degree of branching, granule size and shape as well as crystallization, and on the other hand, the properties resulting in the following features: flow and sorption behavior, pastification temperature, viscosity, thickening performance, solubility, paste structure, transparency, heat, shear and acid resistance, tendency to retrogradation, capability of gel formation, resistance to freezing/thawing, capability of complex formation, iodine binding, film formation, adhesive strength, enzyme stability, digestibility and reactivity. The most remarkable feature is viscosity.

Moreover, the modified starch obtained from the plant cells of the invention may be subjected to further chemical modification, which will result in further improvement of the quality for certain of the above-described fields of application. These chemical modifications are principally known to the person skilled in the art. These are particularly modifications by means of

-   -   acid treatment     -   oxidation and     -   esterification (formation of phosphate, nitrate, sulphate,         xanthate, acetate and citrate starches. Further organic acids         may also be used for esterification.)     -   formation of starch ethers (starch alkyl ether, O-allyl ether,         hydroxylalkyl ether, O-carboxylmethyl ether, N-containing starch         ethers, S-containing starch ethers)     -   formation of branched starches

formation of starch graft polymers.

The invention also relates to propagation material of the plants of the invention, such as seeds, fruits, cuttings, tubers or root stocks, wherein this propagation material contains plant cells of the invention.

Deposits

The plasmids produced and/or used within the framework of the present invention have been deposited at the internationally acknowledged deposit “Deutsche Samnlung von Mikroorganismen (DSM)” in Braunschweig, Federal Republic of Germany, according to the requirements of the Budapest treaty for international acknowledgment of microorganism deposits for patenting (deposit number; deposition date): plasmid pBinAR Hyg (DSM 9505) (Oct. 20, 1994) plasmid p33-anti-BE (DSM 6146) (Aug. 20, 1990) plasmid pRL2 (DSM 10225) (Sep. 04, 1995)

Used Media and Solutions Elution buffer: 25 mM Tris pH 8.3 250 mM glycine Dialysis buffer: 50 mM Tris-HCl pH 7.0 50 mM NaCl 2 mM EDTA 14.7 mM β-mercaptoethanol 0.5 mM PMSF Protein buffer: 50 mM sodium phosphate buffer pH 7.2 10 mM EDTA 0.5 mM PMSF 14.7 mM β-mercaptoethanol Lugol solution: 12 g KI 6 g I₂ ad 1.8 l with ddH₂O 20 x SSC: 175.3 g NaCl 88.2 g sodium citrate ad 1000 ml with ddH₂O ph 7.0 with 10 N NaOH 10 x MEN: 200 mM MOPS 50 mM sodium acetate 10 mM EDTA pH 7.0 NSEB buffer: 0.25 M sodium phosphate buffer pH 7.2 7% SDS 1 mM EDTA 1% BSA (w/v)

DESCRIPTION OF THE FIGURES

FIG. 1 shows the plasmid p35S-anti-RL.

Plasmid structure:

-   -   A=fragment A: CaMV 35S promoter, nt 6909-7437 (Franck et al.,         Cell 21 (1980), 285-294)     -   B=fragment B: Asp718 fragment from pRL1 with a length of         approximately 1949 bp         -   Orientation relative to the promoter: anti-sense         -   The arrow indicates the direction of the open reading frame.     -   C=fragment C: nt 11748-11939 of the T-DNA of Ti-plasmid pTiACH5         T-DNA (Gielen et al., EMBO J. 3 (1984), 835-846)

FIG. 2 shows the plasmid pB33-anti-RL

Plasmid structure:

-   -   A=fragment A: B33 promoter of the patatin gene B33 from Solanum         tuberosum (Rocha-Sosa et al., EMBO J. 8 (1989), 23-29)     -   B=fragment B: Asp718 fragment from pRL1 with a length of         approximately 1949 bp         -   Orientation relative to the promoter: anti-sense         -   The arrow indicates the direction of the open reading frame.     -   C=fragment C: nt 11748-11939 of the T-DNA of Ti-plasmid pTiACH5         T-DNA (Gielen et al., EMBO J. 3 (1984), 835-846)

FIG. 3 shows a Brabender curve of a aqueous starch solution, recorded with a Viskograph-E-type Brabender viscograph, which was isolated from non-transformed potato plants of the variety Désirée (see also Example 8). Thereby signifying: Drehm. torque [BE] Brabender unit Temp. temperature A start of pastification B maximum degree of viscosity C start of the 96° C. period D start of the cooling-off period E end of the cooling-off period F end of the end-50° C. period

The blue line indicates the viscosity; the red line stands for temperature.

FIG. 4 shows a Brabender curve of a aqueous starch solution, recorded with a Viskograph-E-type Brabender viscograph, which was isolated from potato plants transformed with the plasmid p35S-anti-RL (see also Example 8). For the meaning of the abbreviations see FIG. 3.

FIG. 5 shows a Brabender curve of a aqueous solution of starch from potatoes transformed with the plasmid pB33-anti-RL (see also Example 8), recorded with a Viskograph-E-type Brabender viscograph. For the meaning of the abbreviations see FIG. 3.

FIG. 6 shows curves of aqueous solutions of starch isolated from potato plants (see also Example 12.), which were recorded with a Rapid Visco Analyser. The red line stands for the temperature; the blue lines 1, 2, 3 and 4 show the viscosities of the following starch solutions:

-   -   Line 1: starch isolated from wildtype plants,     -   Line 2: starch isolated from plants in which only the branching         enzyme was inhibited (cf. Example 1 of patent application         WO92/14827),     -   Line 3: starch isolated from plants in which merely the         concentration of the proteins of the invention had been reduced         (cf. Example 6).     -   Line 4: starch isolated from plants which had been transformed         with the plasmid p35S-anti-RL in combination with the         p35SH-anti-BE plasmid (cf. Example 12).

FIG. 7 shows curves of aqueous solutions of starch isolated from potato plants (see also Example 13), which were recorded with a Rapid Visco Analyser. The red line stands for the temperature; the blue lines 1, 2, 3 and 4 show the viscosities of the following starch solutions:

-   -   Line 1: starch isolated from wildtype plants,     -   Line 2: starch isolated from plants which had solely been         transformed with the plasmid pB33-anti-GBSSI (so-called         waxy-potato),     -   Line 3: starch isolated from plants which had been solely         transformed with the plasmid p35S-anti-RL (cf. Example 6).     -   Line 4: starch isolated from plants which had been transformed         with the plasmid pB33-anti-RL in combination with the plasmid         pB33-anti-GBSSI (cf. Example 13).

FIG. 8 shows the pRL2 plasmid which comprises a full-length cDNA from potato encoding an R1 enzyme.

THE EXAMPLES ILLUSTRATE THE INVENTION

1. Cloning

For cloning in E.coli the vector pBluescriptSK was used.

For plant transformation the gene constructs were cloned into the binary vector pBinAR (Höfgen and Willmitzer, Plant Sci. 66 (1990), 221-230) and B33-Hyg.

2. Bacterial Strains

For the Bluescript vector and for the pBinAR and B33-Hyg constructs use was made of the E.coli strain DH5α (Bethesda Research Laboratories, Gaithersburgh, USA).

The transformation of plasmid in potato plants was carried out by means of the Agrobacterium tumefaciens strain C58C1 pGV2260 (Deblaere et al., Nucl. Acids Res. 13 (1985), 4777:4788).

3. Transformation of Agrobacterium tumefaciens

The DNA transfer was carried out by means of direct transformation according to the method of Höfgen & Willmitzer (Nucleic Acids Res. 16 (1988), 9877). The plasmid DNA of transformed Agrobacteria was isolated according to the method of Birnboim & Doly (Nucleic Acids Res. 7 (1979), 1513-1523) and electrophoretically analyzed after suitable restriction cleavage.

4. Transformation of Potatoes

Ten small leaves of a sterile potato culture (Solanum tuberosum L. cv. Désirée) injured by a scalpel were treated with 10 ml MS medium (Murashige & Skoog, Physiol.

Plant. 15 (1962), 473-497) with 2% sucrose. The medium contained 50 μl of a Agrobacterium tumefaciens overnight-culture grown under selection. After slightly shaking it for 3-5 minutes, another incubation took place in darkness for two days. The leaves were subsequently put on MS medium with 1.6% glucose, 5 mg/l naphthyl acetic acid, 0.2 mg/l benzylaminopurine, 250 mg/l claforan, 50 mg/l kanamycin or 1 mg/l hygromycin B, and 0.80% Bacto Agar for callus induction. After a one-week incubation at 25° C. and 3000 lux the leaves were put on MS-medium with 1.6% glucose, 1.4 mg/l zeatine ribose, 20 mg/l naphthyl acetic acid, 20 mg/l giberellic acid, 250 mg/l claforan, 50 mg/l kanamycin or 3 mg/l hygromycin B and 0.80% Bacto Agar for shoot induction.

5. Radioactive Marking of DNA Fragments

The radioactive marking of DNA fragments was carried out by means of a DNA-Random Primer Labeling Kits by Boehringer (Germany) according to the manufacturer's instructions.

6. Northern Blot Analysis

RNA was isolated from leave tissue according to standard protocols. 50 μg of the RNA was separated on an agarose gel (1.5% agarose, 1× MEN buffer, 16.6% formaldehyde). After the gel run the gel was briefly washed in water. The RNA was transferred to a Hybond N type nylon membrane (Amersham, UK) with 20×SSC by means of capillary blot. The membrane was subsequently baked in vacuum for two hours at 80° C.

The membrane was prehybridized in NSEB buffer for two hours at 68° C. and subsequently hybridized overnight in NSEB buffer in the presence of the radioactively marked probe at 68° C.

7. Plant Maintenance

Potato plants were kept in the greenhouse under the following conditions:

-   -   light period 16 hours at 25000 lux and 22° C.     -   dark period 8 hours at 15° C.     -   atmospheric humidity 60%         8. Determination of the Amylose/Amylopectin Ratio in Starch         Obtained from Potato Plants

Starch was isolated from potato plants according to standard methods and the amylose/amylopectin ratio was determined according to the method described by Hovenkamp-Hermelink et al. (Potato Research 31 (1988) 241-246).

9. Determination of Glucose, Fructose and Sucrose

In order to determine the glucose, fructose and/or sucrose content, small pieces of potato tubers (with a diameter of approx. 10 mm) are frozen in liquid nitrogen and subsequently extracted for 30 min at 80° C. in 0.5 ml 10 mM HEPES, pH 7.5; 80% (vol./vol.) ethanol. The supernatant containing the soluble components is withdrawn and the volume is determined. The supernatant is used for determining the amount of soluble sugars. The quantitative determination of soluble glucose, fructose and sucrose is carried out in a reaction mixture with the following composition:

-   100.0 mM imidazole/HCl, pH 6.9     -   1.5 mM MgCl₂     -   0.5 mM NADP⁺     -   1.3 mM ATP -   10-50 μl sample -   1.0 U glucose-6-phosphate dehydrogenase from yeast

The reaction mixture is incubated at room temperature for 5 minutes. The subsequent determination of sugars is carried out by means of standard photometric methods by measuring the absorption at 340 nm after successive adding of

-   -   1.0 unit of hexokinase from yeast (for determining glucose)     -   1.0 unit of phosphoglucoisomerase from yeast (for determining         fructose) and     -   1.0 unit of invertase from yeast (for determining sucrose).

EXAMPLE 1 The Isolation of Starch Granule-Bound Proteins from Potato Starch

The isolation of starch granule-bound proteins from potato starch has been carried out by means of electroelution in an elution appliance which was constructed analogous to the “Model 422 Electro Eluter” (BIORAD Laboratories Inc., USA) but had a considerably greater volume (approx. 200 ml). 25 g dried starch were dissolved in elution buffer (final volume 80 ml). The starch was derived from potatoes which produce an almost amylose-free starch due to the antisense-expression of a DNA sequence encoding the starch granule-bound starch synthase I (GBSS I) from potato. The suspension was heated to 70-80° C. in a water bath. Subsequently 72.07 g urea was added (final concentration 8 M) and the volume was filled up to 180 ml with elution buffer. The starch dissolved during permanent stirring and acquired a paste-like consistency. The proteins were electroeluted from the solution overnight by means of the elution appliance (100 V; 50-60 mA). The eluted proteins were carefully removed from the appliance. Suspended particles were removed in a brief centrifugation. The supernatant was dialyzed at 4° C. 2 to 3 times for one hour against dialysis buffer. Subsequently, the volume of the protein solution was determined. The proteins were precipitated by adding ammonium sulfate (final concentration 90%), which was done during permanent stirring at 0° C. The precipitated proteins were pelleted by centrifugation and resuspended in protein buffer.

EXAMPLE 2 Identification and Isolation of cDNA Sequences Encoding Starch Granule-Bound Proteins

The proteins isolated according to Example 1 were used for the production of polyclonal antibodies from rabbit, which specifically recognize starch granule-bound proteins.

By means of such antibodies a cDNA expression library was subsequently screened for sequences encoding starch granule-bound proteins, using standard methods.

The expression library was produced as follows:

Poly (A⁺)-mRNA was isolated from potato tubers of the “Berolina” variety. Starting from the poly (A⁺)-mRNA, cDNA was produced according to the Gubler and Hoffmann method (Gene 25 (1983), 263-269), using an Xho I-Oligo d(t)₁₈ primer. This cDNA was cut with Xho I after EcoR I-linker addition and ligated in an oriented manner in a lambda ZAP II vector (Stratagene) cut with EcoR I and Xho I. Approximately 500,000 plaques of a cDNA library constructed in such a way were screened for sequences which were recognized by polyclonal antibodies directed against starch granule-bound proteins.

In order to analyze the phage plaques these were transferred to nitrocellulose filters which had previously been incubated in a 10 mM IPTG solution for 30 to 60 minutes and had subsequently been dried on filter paper. The transfer took place at 37° C. for 3 hours. Subsequently, the filters are incubated at room temperature for 30 minutes in block reagent and washed for 5-10 minutes in TBST buffer. The filters were shaken with the polyclonal antibodies directed against starch granule-bound proteins in a suitable dilution for one hour at room temperature or for 16 hours at 4° C. The identification of plaques expressing a protein which was recognized by the polyclonal antibodies was carried out by means of the “Blotting detection kit for rabbit antibodies RPN 23” (Amersham UK) according to the manufacturer's instructions.

Phage clones of the cDNA library expressing a protein which was recognized by the polyclonal antibodies were further purified by using standard methods.

By means of the in-vivo excision method, E.coli clones were obtained from positive phage clones containing a double-stranded pBluescript plasmid with the corresponding cDNA insertion. After checking the size and the restriction pattern of the insertions a suitable clone, pRL1, was further analyzed.

EXAMPLE 3 Sequence Analysis of the cDNA Insertion of the Plasmid pRL1

From an E.coli clone obtained according to Example 2 the plasmid pRL1 was isolated and a part of the sequence of its cDNA insertion was determined by standard procedures using the didesoxynucleotide method (Sanger et al., Proc. Natl. Acad. Sci. USA 74 (1977), 5463-5467). The insertion has a length of about 2450 bp. A part of the nucleotide sequence as well as the amino acid sequence derived therefrom is indicated under Seq ID No. 3 and under Seq ID No. 4.

A sequence analysis and a sequence comparison with known DNA sequences showed that the sequence indicated under Seq ID No. 3 is new and exhibits no significant homology to DNA sequences known so far. Moreover, the sequence analysis showed that the cDNA insertion is only a partial cDNA in which a part of the coding region at the 5′-end is missing.

EXAMPLE 4 Identification and Isolation of a Complete cDNA Encoding a Starch Granule-Bound Protein from Solanum tuberosum

In order to isolate a complete cDNA corresponding to the partial cDNA insertion of the plasmid pRL1, a further cDNA library was produced. This was a guard-cell-specific cDNA library from Solanum tuberosum which was constructed as follows:

At first epidermis fragments from leaves of “Desirée” variety potato plants were produced essentially according to the Hedrich et al. method (Plant Physiol. 89 (1989), 148), by harvesting approximately 60 leaves of six-weeks-old potato plants kept in the greenhouse. The center nerve was removed from the leaves. The leaves were subsequently crushed in a big “Waring blender” (with a volume of 1 liter) four times in cooled, distilled H₂O on the highest level for 15 seconds each. The suspension was filtered through a nylon sieve with a mesh size of 220 μm (Nybolt, Zurich, Switzerland) and washed in cold distilled water several times. The suspension itself was filtered through a 220 μm nylon sieve and intensely washed with cold distilled water. The residues (epidermis fragments) were crushed in a smaller “Waring blender” (with a volume of 250 ml) four times in distilled water and ice on a lower level for 15 seconds each. The suspension was filtered through a 220 μm nylon sieve and washed intensely with cold distilled water. The epidermis fragments (residues) were microscopically examined for contamination by mesophyl cells. If contamination occurred the crushing step was repeated in a small “Waring blender”.

The disruption of the guard cells of the epidermis fragments was carried out by means of pulverizing in liquid nitrogen in a cooled mortar for approximately two hours. In order to examine the disruption of the guard cells, probes were regularly taken and microscopically examined. After two hours, or if a sufficiently high amount of guard cells had been disrupted, the obtained powder was filled into a reaction tube (with a volume of 50 ml) and resuspended in one volume GTC buffer (Chirgwin et al., Biochem. 18 (1979), 5294-5299). The suspension was centrifuged and the supernatant was filtered through Miracloth (Calbiochem, La Jolla, Calif.). The filtrate was subjected to ultracentrifugation for 16 hours, as described in Glisin et al. (Biochemistry 13 (1974), 2633-2637) and Mornex et al. (J. Clin. Inves. 77 (1986), 1952-1961). After the centrifugation the RNA precipitate was dissolved in 250 μl GTC buffer. The RNA was precipitated by adding 0.05 volumes of 1 M acetic acid and 0.7 volumes of ethanol. The RNA was precipitated by centrifugation and the precipitate was washed with 3 M sodium acetate (pH 4.8) and 70% ethanol. The RNA was briefly dried and dissolved in DEPC treated water.

Poly A⁺-RNA was isolated from the isolated RNA according to standard methods. Starting from the poly(A⁺)-mRNA, cDNA was produced according to the Gubler and Hoffmann method (Gene 25 (1983), 263-269) by means of a Xho I-oligo d(t)₁₈ primer. This cDNA was cut with Xho I after EcoR I-linker addition and ligated in an oriented manner in a lambda ZAP II vector (Stratagene GmbH, Heidelberg, Germany) cut with EcoR I and Xho I. The packaging in phage heads was carried out using the Gigapack II Gold kit (Stratagene GmbH, Heidelberg, Germany) according to the manufacturer's instructions.

From such a cDNA library phage clones hybridizing with the cDNA insertion of the pRL1 plasmid were isolated and purified according to standard methods. By means of the in vivo excision method E.coli clones were obtained from positive phage clones containing, a double-stranded pBluescript plasmid with the corresponding cDNA insertion. After checking the size and the restriction pattern of the insertions, suitable clones were subjected to restriction mapping and sequence analysis. From a suitable clone the plasmid pRL2 (DSM 10225) was isolated which contains a complete cDNA which encodes a starch granule-bound protein from potato.

EXAMPLE 5 Sequence Analysis of the cDNA Insertion of the pRL2 Plasmid

The nucleotide sequence of the cDNA insertion of the pRL2 plasmid was determined as described in Example 3. The insertion has a length of 4856 bp. The nucleotide sequence as well as the amino acid sequence derived therefrom is indicated in Seq ID No. 1 and/or Seq ID No. 2. In the following, the corresponding gene will be called RL-gene.

EXAMPLE 6 The Construction of the Plasmid P35S-Anti-RL and the Introduction of the Plasmid into the Genome of Potato Plants

By means of the restriction endonuclease Asp718 a DNA fragment with an approximate length of 1800 bp was isolated from the pRL1 plasmid. This corresponds to the DNA sequence indicated under Seq ID No. 3 and contains a part of the open reading frame. The fragment was ligated into the binary vector pBinAR cut with Asp718 (Höfgen and Willmitzer, Plant Sci. 66 (1990), 221-230). This is a derivative of the binary vector pBin19 (Bevan, Nucl. Acids Res. 12 (1984), 8711-8721). pBinAR was constructed as follows:

A fragment with a length of 529 bp comprising the nucleotides 6909-7437 of the 35S promoter of the cauliflower-mosaic virus (Franck et al., Cell 21 (1980), 285-294) was isolated from the plasmid pDH51 (Pietrzak et al., Nucl. Acids Res. 14, 5857-5868) as an EcoR. I/Kpn I fragment and ligated between the EcoR I and the Kpn I sites of the pBin19 polylinker. This led to the plasmid pBin19-A.

By means of the restriction endonucleases Pvu II and Hind III a fragment with a length of 192 bp was isolated from the plasmid pAGV40 (Herrera-Estrella et al., Nature 303, 209-213) comprising the polyadenylation signal of gene 3 of the T-DNA of the Ti-plasmid pTiACH5 (Gielen et al., EMBO J. 3, 835-846) (nucleotides 11749-11939). After the addition of Sph I-linkers, to the Pvu I site the fragment was ligated between the Sph I and Hind III sites of pBin19-A. This led to plasmid pBinAR.

By means of restriction and sequence analysis recombinant vectors were identified in which the DNA fragment is inserted in the vector in such a way that a part of the coding region of the cDNA insertion from pRL1 is linked with the 35S promoter in antisense orientation. The resulting plasmid p35S-anti-RL is shown in FIG. 1.

By inserting the cDNA fragment an expression cassette is produced which consists of the fragments A, B and C:

Fragment A (529 bp) contains the 35S promoter of the cauliflower-mosaic virus (CaMV). The fragment comprises the nucleotides 6909 to 7437 of the CaMV (Franck et al., Cell 21 (1980), 285-294).

Apart from flanking regions, fragment B contains a part of the protein-encoding areas of the cDNA insertion from plasmid pRL1. This was isolated as an Asp718 fragment of pRL1 as described above and fused to the 35S promoter in antisense orientation.

Fragment C (192 bp) contains the polyadenylation signal of gene 3 of the T-DNA of the Ti-plasmid pTiACH5 (Gielen et al., EMBO J. 3 (1984), 835-846).

The plasmid p35S-anti-RL has a size of approximately 12.8 kb. The plasmid was transferred into potato plants by means of Agrobacteria-mediated transformation, as described above. From the transformed cells whole plants were regenerated. The transformed plants were cultivated under greenhouse conditions.

By analyzing total RNA in a Northern Blot analysis concerning the disappearance of the transcripts complementary to the cDNA, the success of the genetic modification of the plants was assessed. For this purpose, total RNA was isolated from leaves of transformed plants according to standard methods and subsequently separated electrophoretically on an agarose gel. Then it was transferred onto a nylon membrane and hybridized with a radioactively labelled probe having the sequence indicated under Seq ID No. 1 or a part thereof. In about 5-10% of the transformed plants the band indicating the specific transcript under Seq ID No. 1 was missing in the Northern Blot analysis. The plants were used for analyzing the starch quality.

EXAMPLE 7 The Construction of the Plasmid pB33-Anti-RL and the Introduction of the Plasmid into the Genome of Potato Plants

By means of the restriction endonuclease Asp718, a DNA fragment with an approximate length of 1800 bp, which comprises a part of the open reading frame of the cDNA insertion was isolated from the plasmid pRL1 and was ligated into the vector B33-Hyg which was cut with Asp718. This vector was constructed as follows:

The 35S promoter was removed from the pBinAR Hyg vector (DSM 9505) by means of the restriction endonucleases EcoR I and Asp718. A fragment with a length of about 1526 bp comprising the B33 promoter was isolated from the plasmid p33-anti-BE (DSM 6146) by means of EcoR I and Asp718 and inserted into the pBinAR Hyg vector (DSM 9505) cut with EcoR I and Asp718.

By inserting the cDNA fragment into the Asp718 site of the B33-Hyg plasmid, an expression cassette is produced which consists of the fragments A, B and C as follows (FIG. 4):

Fragment A contains the B33 promoter from Solanum tuberosum (EP 3775 092; Rocha-Sosa et al., EMBO J. 8 (1989), 23-29).

Apart from flanking regions, fragment B contains a part of the protein encoding region of the cDNA insertion from the pRL1 plasmid. This was isolated as an Asp718 fragment from pRL1 as described above and fused to the B33 promoter in B33-Hyg in antisense orientation.

Fragment C (192 bp) contains the polyadenylation signal of gene 3 of the T-DNA of the Ti-plasmid pTiACH5 (Gielen et al., EMBO J. 3 (1984), 835-846).

The plasmid pB33-anti-RL has a size of approximately 12.8 kb. The plasmid was transferred into potato plants by means of Agrobacteria-mediated transformation, as described above. From the transformed cells whole plants were regenerated. The transformed plants were cultivated under greenhouse conditions. By analyzing total RNA in a Northern Blot analysis concerning the disappearance of the transcripts complementary to the cDNA the success of the genetic modification of the plants was assessed. For this purpose, total RNA was isolated from tubers of transformed plants according to standard methods and subsequently separated electrophoretically on an agarose gel. Then it was transferred onto a nylon membrane and hybridized with a radioactively labelled probe showing the sequence indicated under Seq ID No. 1 or a part thereof. In about 5-10% of the transformed plants the band indicating the transcript hybridizing with the cDNA of the invention was missing in the Northern Blot Analysis. From these plants starch was isolated from tubers and analyzed as described in Example 8.

EXAMPLE 8 Analysis of the Transformed Potato Plants

The potato plants transformed according to Example 6 and Example 7 were examined with regard to the properties of the synthesized starch. Analyses were carried out with various lines of the potato plants which had been transformed with the plasmid p35S-anti-RL or the plasmid pB33-anti-RL and which in Northern Blot analysis had not exhibited the band indicating transcripts hybridizing to the DNA sequences of the invention.

a) Determination of the Viscosity of Aqueous Solutions of the Starch

In order to determine the viscosity of the aqueous solutions of the starch synthesized in transformed potato plants, starch was isolated from tubers of plants which had been transformed with the plasmid p35S-anti-RL or the plasmid pB33-anti-RL using standard methods. 30 g of starch were each taken up in 450 ml H₂O and used for analysis in an E viscograph (Brabender OHG Duisburg (Germany)). The appliance was used according to the manufacturer's instructions. In order to determine the viscosity of the aqueous solution of the starch, the starch suspension was first heated from 50° C. to 96° C. at a speed of 3° C. per minute. The temperature was subsequently kept at 96° C. for 30 min. The solution was then cooled from 96° C. to 50° C. at a speed of 3° C. per minute. During the whole process the viscosity was determined. Representative results of such measurements are set forth in the form of graphs in FIGS. 3, 4 and 5, in which the viscosity is shown depending on time. FIG. 3 shows a typical Brabender graph for starch isolated from wildtype-plants of the potatoe variety Désirée. FIGS. 4 and 5 show a typical Brabender graph for starch isolated from potato plants which had been transformed with the plasmid p35S-anti-RL or pB33-anti-RL. From these graphs characteristic values may be deduced.

The characteristic values for wildtype-plants are as follows: TABLE 1 Time Torque Temperature Value [min:sec] [BE] [° C.] A  6:30  60.5 ± 17.7 69.9 ± 0.57 B 11:30 1838.0 ± 161.2 86.0 ± 2.1  C 15:15 1412.0 ± 18.4  96.0 D 45:15 526.0 ± 17.0 96.0 E 60:30 812.0 ± 8.5  50.0 F 70:45 853.0 ± 5.7  50.0

The values represent the average values obtained from two different measurements.

In Table 1 and the following Tables 2 and 3 the abbreviations signify the following: A: start of pastification B: maximun viscosity C: start of 96° C. period D: start of cooling-off time E: end of cooling-off time F: end of the end-50° C. period

For plants which had been transformed with the plasmid p35S-anti-RL (line P2), the characteristic values are the following: TABLE 2 Time Torque Temperature Value [min:sec] [BE] [° C.] A  6:00 50.0 69.0 B 14:00 820.0 93.0 C 15:15 815.0 96.0 D 45:15 680.0 96.0 E 60:30 1150.0 50.0 F 70:45 1200.0 50.0

For plants which had been transformed with the plasmid pB33-anti-RL (line P3), the characteristic values are the following: TABLE 3 Time Torque Temperature Value [mm:sec] [BE] [° C.] A 7:0 31.0 71.0 B 12:45 671.0 88.3 C 15:15 662.0 96.0 D 45:15 607.0 96.0 E 60:30 1063.0 50.0 F 70:45 1021.0 50.0

FIGS. 3, 4 and 5 explicitly show that the starch obtained from transformed plants differs from starch from wildtype plants particularly in that the viscosity increases only very slightly during heating. Thus, during heating the maximum viscosity of the modified starch from transformed plants is more than 50% lower than in the case of wildtype starch.

During cooling, on the other hand, the viscosity of the starch isolated from transformed plants increases more than in the case of wildtype-plants.

b) Determination of the Phosphate Content of the Starch

The phosphate content of the starch was determined by measuring the amount of phosphate bound to the C-6-position of the glucose residues. For this purpose, starch was first degraded by acid hydrolysis and the glucose-6-phosphate content was subsequently determined by means of an enzyme test, as described in the following.

100 mg starch were incubated in 500 μl 0.7 N HCl for 4 hours at 100° C. After acid hydrolysis 10 μl of the reaction were added to 600 μl imidazole buffer (100 mM imidazole, 5 mM MgCl₂, pH 6.9, 0.4 mM NAD⁺). The amount of glucose-6-phosphate in the reaction mixture was determined by conversion with the enzyme glucose-6-phosphate-dehydrogenase. For this purpose, 1 U glucose-6-phosphate-dehydrogenase (from Leuconostoc mesenteroides (Boehringer Mannheim)) was added to the reaction mixture and the amount of produced NADH was determined by measuring the absorption at 340 nm.

The glucose-6-phosphate content of 1 mg starch is indicated in the following table for non-transformed potato plants of the variety Désirée as well as for two lines (P1 (35S-anti-RL); P2(35S-anti-RL)) of transgenic potato plants which had been transformed with the plasmid p35S-anti-RL. TABLE 4 Plants nmol glucose-6-phopsphate/mg starch % Wildtype 12.89 ± 1.34  100 P1 (35S-anti-RL) 2.25 ± 0.41 17.4 P2 (35S-anti-RL) 1.25 ± 0   9.7

The following table shows the glucose-6-phosphate content per milligram starch in potato plants which were transformed with the plasmid pB33-anti-RL, compared to starch from non-transformed plants (S. tuberosum c.v. Désirée). TABLE 5 Plants nmol glucose-6-phosphate/mg starch % Wildtype 9.80 ± 0.68 100  7 4.50 ± 0.73 45.9 37 2.64 ± 0.99 26.9 45 1.14 ± 0.44 11.6 31 1.25 ± 0.49 12.8

The plants 7, 37, 45 and 31 represent independent transformants which had been transformed with the plasmid pB33-anti-RL. Plant 37 represents line P3 for which a Brabender graph is plotted in FIG. 5.

The values show that the phosphate content of the modified starch from transgenic potato plants is at least 50% lower when compared to starch from wildtype plants.

c) Determination of Glucose, Fructose and Sucrose Content of Tubers after Storage at 4° C.

Tubers of plants from various transgenic lines which had been transformed with the antisense-construct p35S-anti-RL as well as tubers of wildtype plants were stored at 4° C. or, respectively, at 20° C. in darkness, for two months. Subsequently, the amounts of glucose, fructose and sucrose were determined as described above. For two transgenic lines the representative values obtained were the following: TABLE 6 Glucose Fructose Sucrose 20° C. 4° C. 20° C. 4° C. 20° C. 4° C. Wildtype 0.84 55.4 0.62 52.8 8.5 13.1 cv Desiree Transgenic 1.12 6.7 0.75 7.8 7.5 10.1 line 15 Transgenic 1.00 6.4 0.75 7.5 6.9 6.9 line 11 The values in the table are indicated in μmol hexose or sucrose/g fresh weight.

From the values of Table 6 it becomes obvious that the accumulation of reducing sugars in the tubers is considerably lower in transgenic plants stored at 4° C. than in wildtype plants.

Altogether the modified starch isolated from transgenic potato plants resembles starch from maize-wildtype plants. However, in comparison it has the advantage that its taste is neutral and that it is therefore more suitable for various uses in the foodstuffs area.

EXAMPLE 9 Expression of the cDNA Insertion of the pRL2 Plasmid in E.coli

(a) Transformation of Bacterial Cells

In order to express the cDNA insertion of the plasmid pRL2 the cells of the E.coli strain DH5α are first transformed with the pACAC plasmid. This plasmid contains a DNA fragment encoding the ADP-glucose-pyrophosphorylase (AGPase) from E.coli, under the control of the lac Z promoter. The fragment had been isolated from the vector pEcA-15 as a DraI/HaeII fragment with a size of about 1.7 kb (see B. Müller-Röber (1992), dissertation, FU Berlin) and after filling in its sticky ends it had been cloned into a pACAC184 vector linearized with HindIII. The expression of AGPase is to cause an increase of the glycogen synthesis in transformed E.coli cells. The cells transformed in such a way will in the following be named E.coli-K1-cells.

In order to determine the enzyme activity of the protein encoded by the cDNA of plasmid pRL2, E.coli-K1-cells were transformed with the pRL2 plasmid. The transformed E.coli cells which contain the pACAC plasmid as well as the pRL2 plasmid will in the following be named E.coli-K2-cells.

The transfer of the plasmid DNA into the bacterial cells was carried out according to the Hanahan method (J. Mol.

Biol. 166 (1983), 557-580). The transformed E.coli cells were plated onto agar culture dishes with the following composition:

YT medium containing 1.5% Bacto agar 50 mM sodium phosphate buffer, pH 7.2   1% glucose 10 μg/ml chloramphenicol in the case of E. coli-K1-cells or 10 μg/ml chloramphenicol and 10 μg/ml ampicillin in the case of E. coli-K2-cells.

Escherichia coli cells of the DH5α strain which had been transformed with the plasmid pRL2+pACAC (E.coli-K2-cells) and also—for control—solely with the pACAC plasmid (E.coli-K1-cells), were raised on agar plates. The formed glycogen of the various cultures was examined with respect to the degree of phosphorylization (at the C-6 position of the glucose molecule), as described in the following.

(b) Isolation of Bacterial Glycogen

In order to isolate bacterial glycogen, the bacteria colony which had grown after transformation was floated from each 6 agar plates (Ø135 mm) with 5 ml YT medium for each plate. The bacterial suspension was centrifuged at 4500×g for 5 minutes. The bacterial precipitate was resuspended in 10 ml YT medium. Disruption of the bacteria was carried out by adding 2 volumes of disruption medium (0.2 N NaOH; 1% SDS) and by incubation at room temperature for 5 minutes. By adding 3 volumes of EtOH abs., incubating at 4° C. for 30 minutes and subsequent centrifuging at 8000×g for 15 minutes, the glycogen was sedimented. Then the precipitate was washed with 100 ml of 70% EtOH and again sedimented by means of a centrifugation step (10 minutes at 8000×g). The washing procedure was repeated four times.

(c) Determination of the Total Glycogen Content

The isolated and sedimented glycogen was first degraded into single glucose molecules by means of acidic hydrolysis (dissolving of the precipitate in 2 ml 0.7 N HCl; incubation for 4 hours at 100° C.). The glucose content of the solution was determined by means of coupled enzymatic reaction of a starch test with a photometer (Kontron) at a wave length of 340 nm according to the manufacturer's (Boehringer Mannheim) instructions.

The reaction buffer contains:

-   -   100 mM MOPS, pH 7.5     -   10 MM MgCl₂     -   2 mM EDTA     -   0.25 mM NADP     -   1 mM ATP     -   1 U/ml glucose-6-phosphate-dehydrogenase     -   2 U/ml hexokinase

Die measurement was carried out at 25° C. with 10 μl glucose solution.

(d) Determination of the Glucose-6-Phosphate Content

In order to determine the content of glucose molecules phosphorylated at the C-6 position, equal amounts of glucose of the various bacterial cultures were used. By adding the same volumes of 0.7 N KOH to the glycogens degraded into its glucose molecules by acidic hydrolysis (as above), the solution was neutralized.

The reaction buffer contains:

-   -   100 mM MOPS, pH 7.5     -   10 mM MgCl₂     -   2 mM EDTA     -   0.25 mM NADP     -   2 U/ml glucose-6-phosphate-dehydrogenase

The measurement was carried out at 25° C. with 100 to 150 μl glucose solution.

(e) Identification of an Enzyme Activity Phosphorylating Bacterial Glycogen

The results of the determination of the phosphate content of the glycogen synthesized in the bacterial cells show that the glycogen of the E.coli cells, which had been transformed with the pACAC+pRL2 plasmids, exhibits a 290+25% increased phosphorylation at the C-6 position of the glucose when comparing with the control reaction (E.coli cells transformed with the pACYC plasmid) (see the following table). glucose-6-phosphase: glucose in E. coli cells glycogen E. coli-K1 1: (4600 ± 1150) E. coli-K2 1: (1570 ± 390)

The degrees of phosphorylation indicated herein are the average value of at least 6 measurements starting from 6 independent transformations and glycogen isolations.

EXAMPLE 10 Integration of the Plasmid p35S-Anti-RL in Combination with the Plasmid P35SH-Anti-BE into the Genome of Potato Plants

The plasmid p35S-anti-RL was constructed as described in Example 6. The plasmid p35SH-anti-BE was constructed as described in the application WO95/07355, Example 3. Both plasmids were sequentially transferred into potato plants by means of the Agrobacterium mediated transformation as described above. For this purpose, the plasmid p35SH-anti-BE was first transformed in potato plants. Whole plants were regenerated and selected for a reduced expression of the branching enzyme gene. Subsequently, the plasmid p35S-anti-RL was transformed into the transgenic plants already showing a reduced expression of the branching enzyme. From the transformed cells transgenic plants were again regenerated and the transformed plants were cultivated under greenhouse conditions. By analyzing total RNA in an RNA Blot analysis with respect to the disappearance of the transcripts complementary to the branching enzyme cDNA or the RL cDNA, the success of the genetic modification of the plants with respect to a highly reduced expression of the branching enzyme gene as well as with respect to a highly reduced expression of the RL gene was assessed. For this purpose, total RNA was isolated from leaves of transformed plants according to the described methods and subsequently separated by means of gel electrophoresis, transferred onto a membrane, hybridized with a radioactively labelled probe showing the sequence indicated under Seq ID No. 1 or a part thereof and then hybridized with a radioactively labelled probe showing the sequence of the branching enzyme cDNA (cf. WO92/14827, Example 1) or a part thereof. In about 5%-10% of the transformed plants the band indicating the specific transcript of the sequence indicated under Seq ID No. 1 as well as the band indicating the specific transcript of the branching enzyme cDNA (cf. WO92/14827) was missing in the RNA Blot analysis. These plants, which were designated R4 plants were used for analyzing the quality of the starch contained in tubers.

EXAMPLE 11 Integration of the Plasmid pB33-Anti-RL in Combination with the Plasmid pB33-Anti-GBSSI into the Genome of Potato Plants

The plasmid pB33-anti-RL was constructed as described in Example 7. The plasmid pB33-anti-GBSSI was constructed as follows:

The DraI/DraI fragment of the promoter region of the patatin class I gene B33 from Solanum tuberosum comprising the nucleotides −1512 to +14 (Rocha-Sosa et al., EMBO J 8 (1989), 23-29) was ligated into the SmaI site of the pUC19 plasmid. From the resulting plasmid the promoter fragment was ligated into the polylinker region of the pBin19 plasmid (Bevan, Nucleic Acids Research 12 (1984), 8711-8721) as an EcoRI/HindIII fragment. Subsequently, the 3′ EcoRI fragment 1181 to 2511 of the GBSSI gene of Solanum tuberosum (Hergersberg, dissertation (1988), University of Cologne) was ligated into the EcoRI site of the resulting plasmid.

Both plasmids were transferred sequentially into potato plants by means of Agrobacterium mediated transformation as described in Example 10. From the transformed cells whole plants were regenerated and the transformed plants were cultivated under greenhouse conditions. By analyzing the complete RNA in a RNA Blot analysis with regard to the disappearance of the transcripts complementary to the two cDNAs, the success of the genetic modification of the plants was assessed. For this purpose, total RNA was isolated from tubers of transformed plants according to standard methods and subsequently separated on agarose gel by means of gel electrophoresis, transferred onto a membrane and hybridized with a radioactively labelled probe showing the sequence indicated under Seq ID No. 1 or a part thereof. Afterwards, the same membrane was hybridized with a radioactively labelled probe having the sequence of the GBSSI gene or a part of this sequence (Hergersberg, dissertation (1988) University of Cologne). In about 5%-10% of the transformed plants the band indicating the transcripts hybridizing to the cDNA of the invention or the GBSSI cDNA were missing in the RNA Blot analysis. From the tubers of these plants, which were designated R3 plants, starch was isolated and analyzed.

EXAMPLE 12 Starch Analysis of R4 Plants

The potato plants transformed according to Example 10 were examined with respect to the properties of the synthesized starch. The analyses were carried out with various lines of the potato plants which had been transformed with the plasmids p35S-anti-RL and p35SH-anti-BE and which did no longer—or only in extremely reduced form—show the bands indicating transcripts hybridizing to the DNA sequences of the invention or to the sequence of the branching cDNA in RNA Blot analysis.

a) Determination of the Viscosity of Aqueous Solutions of the Starch

In order to determine the viscosity of the aqueous solutions of the starch synthesized in transformed potato plants, starch was isolated from tubers of plants which had been transformed with the plasmid p35S-anti-RL and the plasmid p35SH-anti-BE using standard methods. 2 g of starch were each dissolved in 25 ml H₂O and used for analysis with a Rapid Visco Analyser (Newport Scientific Pty Ltd, Investment Support Group, Warriewood NSW 2102, Australia). The equipment was used according to the instructions of the manufacturer. In order to determine the viscosity of the aqueous solution of the starch, the starch suspension was first heated from 50° C. to 95° C. with a speed of 12° C. per minute. The temperature was then kept at 95° C. for 2.5 minutes. Afterwards, the solution was cooled from 95° C. to 50° C. with a speed of 12° C. per minute. During the whole process the viscosity was measured. Representative results of such measurements are set forth in the form of graphs in which the viscosity is shown depending on time. FIG. 6 shows a typical RVA graph for starch isolated from the wildtype-plants of potato of the variety Désirée. Lines 2 and 3 show a typical RVA graph for starch isolated from the tubers of plants which had been transformed with the plasmid p35SH-anti-BE and with the plasmid p35S-anti-RL, respectively. Line 4 shows a typical RVA graph for starch isolated from tubers of plants which had been transformed with plasmid p35SH-anti-BE in combination with plasmid p35S-anti-RL. Line 4 is characterized in that there is no temperature-dependent increase of viscosity.

b) Determination of the Amylose/Amylopectin Ratio

Starch which was isolated from the tubers of transformed potato plants was examined with respect to the ratio of amylose to amylopectin. The plant line R4-1 (shown in line 4 of FIG. 6) exhibited an amylose content of more than 70%. For the plant line R4-3 an amylose value of 27% was measured, whereas the amylose content in wildtype starch of the Désirée variety ranges between 19 and 22%.

EXAMPLE 13 Starch Analysis of R3 Plants

The potato plants transformed according to Example 11 were examined with respect to the properties of the synthesized starch. The analyses were carried out with various lines of the potato plants which had been transformed with the plasmids pB33-anti-RL and pB33-anti-GBSSI and which did no longer—or only in extremely reduced form—show the bands indicating transcripts hybridizing to the DNA sequences of the invention or to the sequence of the GBSSI cDNA in RNA Blot analysis.

a) Determination of the Viscosity of Aqueous Solutions of the Starch

In order to determine the viscosity of the aqueous solution of the starch synthesized in transformed potato plants, starch was isolated from tubers of plants which had been transformed with the plasmid pB33-anti-RL in combination with the plasmid pB33-anti-GBSSI using standard methods. The viscosity was determined by means of a Rapid Visco Analyser according to the method described in Example 12, part a. The results are indicated in FIG. 7. In line 1, FIG. 7 shows a typical RVA graph for starch isolated from the wildtype-plants of the Désirée potato variety. Lines 2 and 3 show typical RVA graphs for starches isolated from potato plants which had been transformed with the plasmid pB33-anti-GBSSI and with the plasmid p35S-anti-RL, respectively. Line 4 shows a typical RVA graph for starch isolated from potato plants which had been transformed with the plasmid pB33-anti-GBSSI in combination with the plasmid pB33-anti-RL. This graph is characterized in that the maximum viscosity and the increase of viscosity at 50° C. are missing. Furthermore, this starch is characterized in that the glue obtained after RVA treatment exhibits almost no retrogradation after incubating at room temperature for several days.

b) Determination of the Amylose/Amylopectin Ratio

Starch which was isolated from the tubers of transformed potato plants was examined with respect to the ratio of amylose to amylopectin. The plant line R3-5 (shown in line 4 of FIG. 7) exhibited an amylose content of less than 4%. For the plant line R3-6 an amylose content of less than 3% was measured. The amylose content in wildtype starch of the Désirée variety ranges between 19 and 22%.

c) Determination of the Phosphate Content of Starch

The phosphate content of the starch was determined by measuring the amount of phosphate bound to the C-6-position of the glucose residues. For this purpose, starch was first degraded by acid hydrolysis and the glucose-6-phosphate content was subsequently determined by means of an enzyme test, as described in the following.

100 mg starch were incubated in 500 μl 0.7 N HCl for 4 hours at 100° C. After acid hydrolysis 10 μl of the reaction mixture were added to 600 μl imidazole buffer (100 mM imidazole, 5 mM MgCl₂, pH 6.9, 0.4 mM NAD⁺). The amount of glucose-6-phosphate in the preparation is determined by conversion with the enzyme glucose-6-phosphate-dehydrogenase. For this purpose, 1 U glucose-6-phosphate-dehydrogenase (from Leuconostoc mesenteroides (Boehringer Mannheim)) was added to the reaction mixture and the amount of produced NADH was determined by measuring the absorption at 340 nm.

The glucose-6-phosphate content per 1 mg starch is indicated in the following table for non-transformed potato plants of the variety Désirée as well as for the R3-5 and the R3-6 line of transgenic potato plants which had been transformed with the plasmid pB33-anti-RL in combination with the plasmid pB33-anti-GBSSI. As a comparison, the value of the starch from the so-called waxy potato (US2-10) which had been transformed with the plasmid pB33-anti-GBSSI, is also indicated. TABLE 7 Plants nmol glucose-6-phosphate/mg starch % Wildtype 9.80 ± 0.68 100 R3-5 1.32 ± 0.10 13 R3-6 1.37 ± 0.15 14 US2-10 10.82 ± 0.42  110 

1-40. (canceled)
 41. A potato starch comprising (a) amylose in an amount of 27% to 60% as measured by the method of Hovenkamp-Hermelink et al. and (b) amylopectin.
 42. The potato starch of claim 41, which has an amylose content of 50% to 60%.
 43. The potato starch of claim 41, which has been further treated chemically. 